CENOZOIC SYNTHEMS OF THE NORTHWESTERN UNITED STATES

Five interregional Cenozoic unconformity-bounded sequences occur in the non-marine rocks of southwestern Montana and central Washington. Paleosols, paleotopography, and angular discordance of strata variably mark the unconformities. To date, identification of the synthems in southwestern Montana is based largely on paleosols, mammalian biostratigraphy, and seismic reflection data. In contrast, identification of synthems in central Washington is based on changes in composition and provenance. Nonetheless, the results from the two areas are enough alike to demonstrate the similarity of Cenozoic sequence stratigraphy across the northwestern USA. Specifically, major interregional unconformities occur at about 55, 37, 30, 20, and 4 Ma. The few differences in the sequence record for these areas include small variances in sequence boundary ages, and some sequence rock compositions. The sequence age shifts may be due to small differences in the timing of geological events, or could result from yet insufficient data from these areas. Sequence rock compositional differences are most pronounced with the comparison of the late Tertiary sequences (sequences that are about 17 Ma to 4 Ma in age). Columbia River Basalt dominates the Washington sequence, while some basalt, numerous tuffs, and clastic strata comprise the time-equivalent southwestern Montana sequence. However, even with these compositional differences, both sequences reflect the immense influence of volcanic activity during this time.

The Cenozoic synthems extend discontinuously from the Great Plains, through Montana, to Oregon and western Washington. The recognition of their great extent will help in understanding the geologic history of the northwestern USA.